Health authorities warned for months that the Zika virus would likely spread to the continental U.S. this summer. On Friday, they said they believe that has now occurred.

Four people in the Miami area—one woman and three men—likely were infected with Zika in early July by mosquito bites, the first to contract the virus that way in any U.S. state, according to federal and Florida officials.

Their illnesses illustrate how...

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